ABSTRACT
This study reports for the first time in India, recovery of L. petauri from clinical samples of farmed rainbow trout (Oncorhynchus mykiss ). The clinal sign of the disease was hemorrhage in eye, fin erosion and melanosis. The isolate was characterized by biochemical tests, PCR and whole-genome sequencing (WGS). Biochemical test and WGS confirmed the identity of bacterium as L. petauri isolate PA1. The genome of L. petauri PA1 (GenBank accession number: JBMKJY000000000) comprises 2,708,129 bp with a GC content of 37.6%, encoding 3,489 genes, including 1,638 core , 859 shell, 934 cloud, and 58 RNA genes (49 tRNAs, 3 ncRNAs, and 6 rRNAs). Average Nucleotide Identity (ANI) of L. petauri PA1 showed 97.39 - 98.55% sequence similarity with other L. petauri . WGS of this bacterium identified 139 virulence and 162 antimicrobial resistance (AMR) genes. The L. petauri PA1 was positive for PCR amplification (465 bp) of the Lichenan permease IIC component gene, a molecular marker of L. petauri . In vitro virulence assay of extracellular protease (ECPs) of L. petauri PA1 in epithelioma papulosum cyprini (EPC) cells shows cytotoxicity and cell death within 60 h of exposure. In vivo virulence test of bacterium via intraperitoneal injection (2.9 × 106 CFU per fish) in rainbow trout (average body weight: 20.5 ± 4.37 g ) reproduced clinical signs of lactococcosis , with histopathology confirming multi-organ damage. The LD₅₀ value of L. petauri PA1 in rainbow trout was 3.2 × 106 CFU per fish. Antimicrobial susceptibility test (AST) shows that the isolate is sensitive to majority of the antibiotics. Our findings establish L. petauri as a virulent pathogen in aquaculture, emphasizing the need for proactive surveillance and disease management strategies.
Keywords: Fish, Bacteria, Emerging pathogen, Lactococcus, Aquaculture